The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In an automatic transmission field, a research for an increase of the number of shift-stages is generally performed for a technology for improving fuel efficiency of a vehicle and maximizing drivability, and a recent increase in oil price causes unlimited competition for the improvement of the fuel efficiency.
As a result, in the case of an engine, a research capable of reducing weight and improving fuel efficiency by down-sizing the engine has been conducted, and in the case of an automatic transmission, a research capable of simultaneously securing drivability and fuel efficiency competitiveness by the increase of the number of shift-stages of the automatic transmission has been conducted.
However, as the number of shift-stages of the automatic transmission is increased, the number of internal parts, particularly, the number of planetary gear sets is increased, which increases an entire length of the transmission, thereby causing mountability, production cost, weight, transfer efficiency, etc., to be deteriorated.
In this aspect, the automatic transmission is configured to implement a shift of 8-speed or more and is mounted in the vehicle, and a research and development into a planetary gear train capable of implementing a shift-stage of 8-speed or more has been continuously desired.
However, a general automatic transmission of 8-speed or more generally includes 3 to 4 planetary gear sets and 5 to 7 control elements (friction elements), and in this case, since the entire length is increased, mountability may be deteriorated.
Accordingly, in order to increase the number of shift-stages of the automatic transmission, a double row structure in which the planetary gear set is disposed on the planetary gear set is recently adopted, or a dog clutch instead of a wet control element is recently applied, but in this case, we have discovered that there is a problem that an applicable structure is limited, and deterioration of shift sensitivity is involved due to the application of the dog clutch.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.